The present invention is generally related to wireless communication systems. More particularly, the present invention is related to a channel scanning method for use in a wireless communication system.
In wireless communication systems a mobile station establishes radio communication with a fixed station. The mobile station may also be referred to as a handset or portable part (PP) and is typically battery-powered for mobility. The fixed station may also be referred to as a radio fixed part (RFP) or base station. The fixed station may be part of a network such stations including a network controller.
Radio communication occurs over a plurality of channels. In a time division multiple access (TDMA) system, a plurality of carriers transmits information during time slots of predetermined duration. Each one-way channel is defined by a time slot and a carrier. A two-way or duplex link is defined by two non-adjacent time slots on the same carrier.
Channels must be selected and may be changed according to predetermined rules. As the mobile station moves, it may experience noise or other interference. As the mobile station moves from the vicinity of one base station to the vicinity of another base station, it may have to begin communication with the other base station on another channel, in a process called handover. To select and change channels requires a knowledge of available channels.
In some wireless systems, channel selection is assigned to the mobile station, rather than the system. This is true in handsets operating in unlicensed personal communication system (PCS) bands at 1920-1930 MHz under regulations of the U.S. Federal Communications Commission (FCC). The handset uses an otherwise idle time slot to search the radio environment for activity. The result of this scanning is stored in a channel history database. When interference is encountered or the handset moves around the system coverage area, the handset will dynamically select a better channel from the channel history database. A channel switch or channel handover will then occur.
This process is known as dynamic channel selection (DCS). An important sub-process of DCS is channel scanning. The handset channel scanning process is performed both during an active call and during standby. As a result, the channel scanning process is a major determining factor of handset battery life.
The need to scan the radio environment creates a design tradeoff. By scanning as frequently as possible, the most current radio environment information can be maintained. As a result, a more reliable channel database is maintained for proper channel selection. However, the negative side of a high channel scan rate is high power consumption and concomitant reduced battery life of the handset.
Conventional channel scanning methods use a fixed scan interval. In one implementation, a complete channel scan is performed every 128 frames, where each frame includes 24 time slots. This conventional method faces limitation when the radio environment changes rapidly, for example, when the handset user moves quickly from one cell coverage area to another coverage area or the interference level rises suddenly. In such an instance, the scan interval may be too long to keep up with the rapidly changing environment. The channel information and the channel history database might not reflect the actual channel conditions at a particular time. The consequence is a delay in channel selection, noisy communication including high bit error rate and even dropped calls. On the other end, when the radio environment changes very slowly the fixed scan rate may be too high. For example, when the handset user and handset are stationary in a quiet office, the channel data does not change much from scan to scan. In that case, the scan rate is too high and channel scanning consumes battery power unnecessarily.
Accordingly, there is a need for an improved channel scanning technique which balances the need for current radio environment information and the need for reduced battery consumption.